Superhard constructions &amp; methods of making same

ABSTRACT

A superhard polycrystalline construction ( 1 ) comprises a body ( 12 ) of polycrystalline superhard material having a cutting face ( 14 ) and a substrate ( 10 ) bonded to the body of polycrystalline superhard material along an interface ( 18 ). The substrate comprises a substrate body and a first end surface ( ) 20  forming the interface, the first end surface of the substrate comprising a projection extending from the body of the substrate into the body of superhard material towards the cutting face. The projection has an outer peripheral surface around which the body of polycrystalline superhard material extends. The body of polycrystalline superhard material has a thickness (h) from the cutting face along the peripheral side edge ( 13 ) to the interface with the substrate of at least around 4 mm and at least a portion of the projection has a thickness measured in a plane extending along the longitudinal axis of the construction of at least around 4 mm. There is also disclosed a method of making such a superhard polycrystalline construction.

FIELD

This disclosure relates to superhard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures attached to a substrate, and tools comprising the same, particularly but not exclusively for use in rock degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline superhard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of super hard tool inserts may be limited by fracture of the super hard material, including by spalling and chipping, or by wear of the tool insert.

Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and a super hard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the super hard material layer is typically polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable product TSP material such as thermally stable polycrystalline diamond, the superhard layer bonded to the substrate in a PCD cutter element typically having a maximum thickness from the interface with the substrate to the working surface of around 2 mm.

Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material or ultra hard material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its reprecipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent-catalysts for PCD sintering.

Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with a superhard material layer such as PCD or PCBN, diamond particles or grains or CBN grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains or CBN grains occurs, forming a polycrystalline superhard diamond or polycrystalline CBN layer.

In some instances, the substrate may be fully cured prior to attachment to the superhard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the superhard material layer.

Ever increasing drives for improved productivity in the earth boring field place ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.

Cutting elements or tool inserts comprising PCD material are widely used in drill bits for boring into the earth in the oil and gas drilling industry. Rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite.

The most wear resistant grades of PCD and PCBN used in cutters usually fail by spalling resulting in a catastrophic fracture of the cutter before it has worn out. Spalling is considered to be caused by a crack propagating from working area to the top free surface of the cutting tool. During the use of these cutters, cracks grow until they reach a critical length at which catastrophic failure occurs, namely, when a large portion of the PCD o PCBN breaks away in a brittle manner. Catastrophic failure of a component or structure indicates that crack grew to reach the “critical crack length” of the given structural material. The “critical crack length” is the acceptable length of crack beyond which the propagation of the crack becomes uncontrollable leading to catastrophic failure independently of the remaining non-working area of the component. The long, fast growing cracks encountered during use of conventionally sintered PCD and PCBN can therefore result in shorter tool life.

Furthermore, despite their high strength, polycrystalline diamond (PCD) and PCBN materials are usually susceptible to impact fracture due to their low fracture toughness. Improving fracture toughness without adversely affecting the material's high strength and abrasion resistance is a challenging task.

There is therefore a need for a superhard composite that has good or improved abrasion, fracture and impact resistance and a method of forming such composites.

SUMMARY

Viewed from a first aspect there is provided a superhard polycrystalline construction comprising:

-   -   a body of polycrystalline superhard material having a cutting         face; and     -   a substrate bonded to the body of polycrystalline superhard         material along an interface;     -   the construction having a central longitudinal axis extending         therethrough and a peripheral side edge; wherein:     -   the substrate comprises a substrate body and a first end surface         forming the interface, the first end surface of the substrate         comprising a projection extending from the body of the substrate         into the body of superhard material towards the cutting face,         the projection having an outer peripheral surface, the body of         polycrystalline superhard material extending around the         peripheral outer surface of the projection;     -   wherein the body of polycrystalline superhard material has a         thickness from the cutting face along the peripheral side edge         to the interface with the substrate of at least around 4 mm; and     -   wherein at least a portion of the projection has a thickness         measured in a plane extending along the longitudinal axis of the         construction of at least around 4 mm.

Viewed from a second aspect there is provided a method of forming a superhard polycrystalline construction, comprising:

-   -   providing a first mass of particles or grains of superhard         material;     -   admixing the first mass of particles or grains with a binder         material to form a green body;     -   placing the green body in contact with a pre-formed substrate to         form a pre-sinter assembly, the pre-formed substrate having a         longitudinal axis and comprising a body portion and a         projection, the projection extending at least in part from the         body portion by around 4 mm or greater as measured in a plane         parallel to the longitudinal axis of the substrate;     -   treating the pre-sinter assembly in the presence of a         catalyst/solvent material for the superhard grains at an         ultra-high pressure of around 5.5 GPa or greater and a         temperature to sinter together the grains of superhard material         to form a polycrystalline superhard construction comprising a         body of polycrystalline superhard material having a cutting         face; the substrate being bonded to the body of polycrystalline         superhard material along an interface; wherein the projection         extends from the body of the substrate into the body of         superhard material towards the cutting face, the body of         polycrystalline material extending around the projection; and         wherein the body of polycrystalline material has a thickness         from the cutting face along a peripheral side edge of the         construction to the interface with the substrate of at least         around 4 mm.

Viewed from a further aspect there is provided a tool comprising the superhard polycrystalline construction defined above, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

The tool may comprise, for example, a drill bit for earth boring or rock drilling, a rotary fixed-cutter bit for use in the oil and gas drilling industry, or a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.

Viewed from another aspect there is provided a drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of an example superhard cutter element for a drill bit for boring into the earth;

FIGS. 2a to 2e are schematic cross-sections of example superhard cutter elements with differing interfaces between the superhard body and substrate attached thereto;

FIGS. 3a and 3b are schematic cross-sections of further example superhard cutter elements in which the superhard bodies are formed of regions comprising differing grain sizes and/or compositions, the interface between the substrate and the superhard body being spaced from the working surface of the cutter element in both examples;

FIGS. 4a and 4b are schematic cross-sections of further example superhard cutter elements in which the superhard bodies are formed of regions comprising differing grain sizes and/or compositions, the interface between the substrate and the superhard body extending to the working surface of the cutter element in both examples;

FIGS. 5a to 5c are perspective view from above of three example substrate portions showing the shaped end of the substrate which is to form the interface with a superhard layer, prior to attachment to a superhard layer;

FIG. 6 is a schematic cross-section through an example superhard cutter element showing the boundary between a leached portion and an unleached portion of the superhard layer;

FIG. 7a is a schematic cross-section through a conventional superhard cutter element showing wear into the substrate through use;

FIG. 7b is a schematic cross-section through an example superhard cutter element showing wear remaining in the superhard body after use; and

FIG. 8 is a plot showing the results of a vertical borer test comparing two conventional leached PCD cutter elements, and an example PCD cutter element.

The same references refer to the same general features in all the drawings.

DESCRIPTION

As used herein, a “superhard material” is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of superhard materials.

As used herein, a “superhard construction” means a construction comprising a body of polycrystalline superhard material. In such a construction, a substrate may be attached thereto.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline superhard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. In one embodiment of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. As used herein, “interstices” or “interstitial regions” are regions between the diamond grains of PCD material. In embodiments of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

A “catalyst material” for a superhard material is capable of promoting the growth or sintering of the superhard material.

The term “substrate” as used herein means any substrate over which the superhard material layer is formed. For example, a “substrate” as used herein may be a transition layer formed over another substrate.

As used herein, the term “integrally formed” means regions or parts are produced contiguous with each other and are not separated by a different kind of material.

Components comprising PCBN are used principally for machining metals. PCBN material comprises a sintered mass of cubic boron nitride (cBN) grains. The cBN content of PCBN materials may be at least about 40 volume %. When the cBN content in the PCBN is at least about 70 volume % there may be substantial direct contact among the cBN grains. When the cBN content is in the range from about 40 volume % to about 60 volume % of the compact, then the extent of direct contact among the cBN grains is limited. PCBN may be made by subjecting a mass of cBN particles together with a powdered matrix phase, to a temperature and pressure at which the cBN is thermodynamically more stable than the hexagonal form of boron nitride, hBN. PCBN is less wear resistant than PCD which may make it suitable for different applications to that of PCD.

In an embodiment as shown in FIG. 1, a cutting element 1 includes a substrate 10 with a layer of superhard material 12 formed on the substrate 10. The substrate 10 may be formed of a hard material such as cemented tungsten carbide. The superhard material 12 may be, for example, polycrystalline diamond (PCD), a thermally stable product such as thermally stable PCD (TSP), or polycrystalline cubic boron nitride (PCBN). The cutting element 1 may be mounted into a bit body such as a drag bit body (not shown) and may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth.

The exposed surface of the superhard material opposite the face which forms the interface with the substrate, forms the cutting face 14 of the cutter element, that is, the surface which, along with its edge 16, performs the cutting in use.

At one end of the substrate 10 is an interface surface 18 that forms an interface with the superhard material layer 12 which is attached thereto at this interface surface. As shown in the embodiment of FIG. 1, the substrate 10 is generally cylindrical and has a peripheral top edge 20 and a peripheral surface 22.

As used herein, a PCD or PCBN grade is a PCD or PCBN material characterised in terms of the volume content and size of diamond grains in the case of PCD or cBN grains in the case of PCBN, the volume content of interstitial regions between the grains, and composition of material that may be present within the interstitial regions. A grade of superhard material may be made by a process including providing an aggregate mass of superhard grains having a size distribution suitable for the grade, optionally introducing catalyst material or additive material into the aggregate mass, and subjecting the aggregated mass in the presence of a source of catalyst material for the superhard material to a pressure and temperature at which the superhard grains are more thermodynamically stable than graphite (in the case of diamond) or hBN (in the case of CBN), and at which the catalyst material is molten. Under these conditions, molten catalyst material may infiltrate from the source into the aggregated mass and is likely to promote direct intergrowth between the diamond grains in a process of sintering, to form a polycrystalline superhard structure. The aggregate mass may comprise loose superhard grains or superhard grains held together by a binder material. In the context of diamond, the diamond grains may be natural or synthesised diamond grains.

Different grades of superhard material such as polycrystalline diamond may have different microstructures and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K₁C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

In the context of PCD, the PCD grades may comprise interstitial regions filled with material comprising cobalt metal, which is an example of catalyst material for diamond.

The polycrystalline superhard structure 12 shown in the cutter element of FIG. 1 may comprise, for example, one or more PCD grades.

FIGS. 2a to 2e are schematic cross-sections through five embodiments of example polycrystalline superhard cutter elements 1. The five examples all comprise a substrate 10 extending to a distance t from the cutting face 14 of the polycrystalline superhard structure 12, the polycrystalline superhard structure 12 having a thickness h as measured from the cutting face 14 of the polycrystalline superhard structure 12 along the barrel 13 thereof to the interface with the substrate 10, the barrel 13 being the peripheral side edge of the cutter element 1. In these embodiments shown in FIGS. 2a to 2e , the thickness h is preferably greater than or equal to around 4 mm. Furthermore, in these embodiments, the thickness t is preferably less than or equal to around 0.5 mm. In these embodiments, the polycrystalline superhard layer 12 extends over the substrate portion at the cutting face 14 and this may be advantageous as the substrate 10 is thereby protected from chemical erosion and abrasion during application and also from chemical attack in the event that the cutter element 1 is subjected to a treatment such as acid leaching after sintering.

In some embodiments, and in particular those where the planar central section 26 of the substrate extends to and forms part of the cutting face 14, the cutting face 14 or a portion thereof may be protected against erosion, corrosion or chemical degradation by attaching or spraying for example a layer of resistant polymer, oxide, paint, composite materials, onto the surface. The protective layer(s) may be formed during pre-composite assembly and bonded on to the cutter surface during HPHT sintering. Alternatively, the protective layer(s) may be attached to the cutter surface after sintering and processing and adhered thereto by surface interaction.

The five embodiments of FIGS. 2a to 2e differ in the shape of the end face of the substrate portion 10 which forms the interface 18 with the polycrystalline superhard structure 12. In the example shown in FIG. 2a , the end face of the substrate portion 10 which forms the interface 18 is dome shaped with the highest point 24 of the dome along the longitudinal axis of the cutter element being spaced from the cutting face 14 by a distance t along the longitudinal axis of the cutter.

In the example shown in FIG. 2b , the end face of the substrate portion 10 which forms the interface 18 has a planar, coaxially located central section 26 which, at the end face is circular in cross section having a diameter d. This planar section 26 forms the furthest point of the interface 18 from the body of the substrate and is spaced from the cutting face 14 by a distance t along the longitudinal axis of the cutter element 1. The diameter d of the planar central section 26 of the substrate which forms part of the interface 18 with the superhard layer is less than the diameter D of the cutter element 1. The surface 28 of the substrate extending from the peripheral edge of the planar central section 26 to the peripheral side edge or barrel 13 of the cutter element 1 at a distance h along the barrel 13 from the cutting face 14, is concavely curved such that the superhard layer 12 extends across the planar central section 26.

The example shown in FIG. 2c differs from that shown in FIG. 2b in that the surface 28 of the substrate extending from the peripheral edge of the planar central section 26 to the peripheral side edge of the cutter element 1 is not curved but is instead sloped, that is shown by the inclined plane depicted in cross section in FIG. 2c , the substrate thereby comprising a truncated cone at the interface end projecting from the body of the substrate and extending through the layer of superhard material towards the cutting face 14.

The example in FIG. 2d differs from that of FIG. 2c in that the surface of the substrate portion extending from the planar central section 26 extends in a plane parallel to the central longitudinal axis of the cutter element for a length equal to (h-t) and then radially outward to the peripheral side edge, that is to the barrel 13, of the cutter element 1. Thus, the substrate includes a coaxially extending cylindrical portion extending within the body of superhard material, from the body of the substrate towards the cutting face 14 of the superhard layer. The example in FIG. 2e differs from that in FIG. 2d in that the surface 28 of the substrate extending from the planar central section 26 is inclined at an angle A to the plane parallel to the plane through which the longitudinal axis of the cutter element extends, the height of the planar central section being denoted by h′ and the radial length of the portion extending from the planar central section radially to the barrel 13 of the cutter element is denoted by B. The intersection 29 between the planar central section 26 and the sides thereof and between those sides and the radially extending portion may be curved or meet at a point. Thus the substrate comprises a truncated cone extending from the body of the substrate towards the cutting face.

In these embodiments, the angle A may be between about 0 to about 15 degrees, and in some embodiments around 5 degrees or less, and the distance B may be, for example, between about 0 to about 3 mm, and in some embodiments around 2 mm or less.

FIGS. 3a and 3b show further examples of cutter elements similar to that shown in FIG. 2e but with the intersections 29 being points and an interlayer 30 being located between either a portion of the substrate and the superhard layer (as shown in FIG. 3a ) or forming the entire interface between the substrate and the superhard layer (as shown in FIG. 3b ). The interlayer 30 may be comprised of, for example, a different grade of superhard material to that of the superhard layer 12, and/or, it may be a different composition to the superhard layer 12.

In the embodiment shown in FIG. 3a , the interlayer 30 is positioned between the superhard layer 12 and the substrate 10 and extends about at least a portion of the planar central section 26. In this embodiment, the interlayer 30 does not extend to the full height of the planar central section surface but extends annularly therearound and is spaced from the cutting face 14. In the example of FIG. 3b , the interlayer 30 does extend over all of the surface features of the substrate and spaces the superhard layer 12 from the substrate 10. In this embodiment, it is the interlayer 30 covering the planar central section that, at its highest point, is spaced a distance t from the cutting face 14, rather than the uppermost features of the substrate 10 itself.

The examples of FIGS. 4a and 4b differ from those shown in FIGS. 3a and 3b respectively in that in the examples of FIGS. 4a and 4b the planar central section 26 of the substrate 10 extends to and forms part of the cutting face 14. The length of the cutter element from the base of the substrate to the cutting face as measured along the longitudinal axis of the cutter element is denoted by H1 and the height of the central section 26 as measured in a plane parallel to the central longitudinal axis of the cutter element along the barrel (side edge) of the cutter element is denoted by H2.

Multiple interlayers of different grain size and composition may be included which, in some embodiment, may be substantially parallel to one another. One or more of such interlayers may comprise a mixture of WC and diamond powders, a mixture of cBN and diamond powders, a mixture of refractory metals and super-hard (such as W, V, Mo) material powders, or any combination thereof. Whilst not wishing to be bound by a particular theory, it is believed that such interlayers adjacent to the substrate may eliminate the sudden change in CTE between the substrate and the superhard layer and thereby assist in inhibiting cracking and/or delamination of the sintered superhard layer from the substrate by minimising residual stress between layers of different compositions.

When subjected to post-sintering treatments such as acid leaching to remove residual binder from interstices between the superhard grains, the layers may introduce different leaching rates in the cutter resulting in preferential leaching profiles to be achieved.

FIGS. 5a to 5c show three examples of the shapes of possible substrate portions which may form the interface with either an interlayer 30 or superhard layer 10 (not shown). In FIGS. 5a to 5c , the planar central portions 26 differ in shape from those of the other figures in that they have the general shape obtained by truncating the space between three tangent circles forming a coaxially located projection from the body of the substrate with a planar free surface position. In FIG. 5b , the projection from the substrate to the planar free surface thereof is of substantially constant cross-sectional area and extends to the barrel 13 of the cutter element 1. In FIG. 5a the cross-sectional area of the planar free surface of the projection from the substrate is smaller than at the base thereof, and the surfaces extending between the features of the projection to the barrel 13 are curved concavely.

In FIG. 5c , this differs from the substrate shown in FIG. 5a in that the projection extends to a height from the body of the substrate before decreasing in cross-sectional area to the planar end surface thereof whilst maintaining the same general shape. The surface joining the top and bottom of the projection is curved concavely.

The projection from the substrate in the examples of FIGS. 5a to 5c is therefore non-conical and non-axisymmetric in shape and divides the cutting face 14 into three segments which may then be filled by the polycrystalline superhard material which is separated from adjacent segments by a core of tougher substrate material and spokes extending towards the barrel of the cutter. The advantage of these constructions may be that the cutter is rotatable after use such that a different cutting edge may be presented to the surface to be cut and also the segments act to confine damage to a limited area of the cutter during use.

FIG. 6 is a schematic cross-section of the cutter of FIG. 2e which has been subjected to a post sintering treatment such as acid leaching to remove residual binder from interstices between the superhard grains forming the polycrystalline superhard layer 12. The boundary between the leached and unleached portions is denoted by reference numeral 36 and follows the same general shape of the interface between the substrate 10 and the superhard layer 12. In this example, it may be possible to control the leaching profile such that there is a greater leached volume denoted by L in FIG. 6 than unleached volume of superhard material extending in from the barrel of the cutter element and the cutting face 14 of the cutter element may remain unleached or be leached to a depth of, for example, around 200 microns or less from the cutting face 14. Also, given the height of the superhard layer 12, it may be possible to leach the barrel region 13 of the cutter element 1 to a depth of at least around 3.5 mm and in some embodiments to a depth of around 4.5 mm or greater.

FIG. 7a is a schematic cross-section through a conventional PCD cutter 37 formed of a substrate 38 attached to a layer of PCD material 39 showing wear into the substrate 38 through use. It will be seen that the wear flat 40 has progressed through both the PCD layer 39 and the substrate 37.

FIG. 7b is a schematic cross-section through an example PCD cutter element showing wear remaining in the PCD body after use. The cutter shown in FIG. 7a is that of FIG. 3a and it will be seen that the wear flat 40 is retained in the layer of superhard material 12 and does not extend into the substrate 10 attached thereto.

Thus embodiments of the invention may enable the wear scar surface of the cutter to be maintained in the layer of superhard material which is advantageous as the wear scar surface may thereby be composed of homogeneous material and hence provide uniform friction across the wear scar surface. Having heterogeneous material across the wear scar surface as in the conventional cutter shown in FIG. 7a will result in the wear scar surface being formed of materials having different coefficients of friction which may contribute to crack initiation near the wear scar leading to reduced performance of the cutter and increased susceptibility of the cutter to failure through spalling.

FIG. 8 is a plot showing the results of a vertical borer test comparing two conventional leached PCD cutter elements, and an example PCD cutter element.

The grains of superhard material may be, for example, diamond grains or particles, or for example, cBN grains or particles. In the starting mixture prior to sintering they may be, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of superhard grains and a fine fraction of superhard grains. In some embodiments, the coarse fraction may have, for example, an average particle/grain size ranging from about 10 to 60 microns. By “average particle or grain size” it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the “average”. The average particle/grain size of the fine fraction is less than the size of the coarse fraction, for example between around 1/10 to 6/10 of the size of the coarse fraction, and may, in some embodiments, range for example between about 0.1 to 20 microns.

In some embodiments, the weight ratio of the coarse fraction to the fine fraction ranges from about 50% to about 97% coarse superhard grains and the weight ratio of the fine fraction may be from about 3% to about 50%. In other embodiments, the weight ratio of the coarse fraction to the fine fraction will range from about 70:30 to about 90:10.

In further embodiments, the weight ratio of the coarse fraction to the fine fraction may range for example from about 60:40 to about 80:20.

In some embodiments, the particle size distributions of the coarse and fine fractions do not overlap and in some embodiments the different size components of the compact are separated by an order of magnitude between the separate size fractions making up the multimodal distribution.

Some embodiments consist of a wide bi-modal size distribution between the coarse and fine fractions of superhard material, but some embodiments may include three or even four or more size modes which may, for example, be separated in size by an order of magnitude, for example, a blend of particle sizes whose average particle size is 20 microns, 2 microns, 200 nm and 20 nm.

Sizing of diamond particles/grains into fine fraction, coarse fraction, or other sizes in between, may be through known processes such as jet-milling of larger diamond grains and the like.

In embodiments where the superhard material is polycrystalline diamond material, the diamond grains used to form the polycrystalline diamond material may be natural or synthetic.

In some embodiments, the polycrystalline superhard material is PCBN and the superhard particles or grains comprise cBN.

In some embodiments, the binder catalyst/solvent used to assist in the bonding of the grains of superhard material such as diamond grains, may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some embodiments, the binder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in composition and, thus, may be include any of the Group IVB, VB, or VIB metals, which are pressed and sintered in the presence of a binder of cobalt, nickel or iron, or alloys thereof. In some embodiments, the metal carbide is tungsten carbide.

The cutter of FIG. 1 may be fabricated, for example, as follows.

As used herein, a “green body” is a body comprising grains to be sintered and a means of holding the grains together, such as a binder, for example an organic binder.

Embodiments of superhard constructions may be made by a method of preparing a green body comprising grains or particles of superhard material, non-reactive phase and a binder, such as an organic binder. The green body may also comprise catalyst material for promoting the sintering of the superhard grains. The green body may be made by combining the grains or particles with the binder/catalyst and forming them into a body having substantially the same general shape as that of the intended sintered body, and drying the binder. At least some of the binder material may be removed by, for example, burning it off. The green body may be formed by a method including a compaction process, an injection process or other methods such as molding, extrusion, deposition modelling methods.

The substrate is preferably pre-formed. In some embodiments, the substrate may be pre-formed by pressing the green body of grains of hard material such as tungsten carbide into the desired shape, including the interface features at one free end thereof, and sintering the green body to form the substrate element. In an alternative embodiment, the substrate interface features may be machined from a sintered cylindrical body of hard material, to form the desired geometry for the interface features. The substrate may, for example, comprise WC particles bonded with a catalyst material such as cobalt, nickel, or iron, or mixtures thereof. A green body for the superhard construction, which comprises the pre-formed substrate and the particles of superhard material such as diamond particles or cubic boron nitride particles, may be placed onto the substrate, to form a pre-sinter assembly which may be encapsulated in a capsule for an ultra-high pressure furnace, as is known in the art. In particular, the superabrasive particles, for example in powder form, are placed inside a metal cup formed, for example, of niobium, tantalum, or titanium. The pre-formed substrate is placed inside the cup and hydrostatically pressed into the superhard powder such that the requisite powder mass is pressed around the interface features of the preformed carbide substrate to form the pre-composite. The pre-composite is then outgassed at about 1050 degrees C. The pre-composite is closed by placing a second cup at the other end and the pre-composite is sealed by cold isostatic pressing or EB welding. The pre-composite is then sintered to form the sintered body of superhard material bonded to the substrate along the interface therewith.

The substrate may provide a source of catalyst material for promoting the sintering of the superhard grains. In some embodiments, the superhard grains may be diamond grains and the substrate may be cobalt-cemented tungsten carbide, the cobalt in the substrate being a source of catalyst for sintering the diamond grains. The pre-sinter assembly may comprise an additional source of catalyst material.

In one example, the method may include loading the capsule comprising a pre-sinter assembly into a press and subjecting the green body to an ultra-high pressure and a temperature at which the superhard material is thermodynamically stable to sinter the superhard grains. In some embodiments, the green body may comprise diamond grains and the pressure to which the assembly is subjected is at least about 5 GPa and the temperature is at least about 1,300 degrees centigrade. In some embodiments, the pressure to which the assembly may be subjected is around 5.5-6 GPa, but in some embodiments it may be around 7.7 GPa or greater. Also, in some embodiments, the temperature used in the sintering process may be in the range of around 1400 to around 1500 degrees C.

A version of the method may include making a diamond composite structure by means of a method disclosed, for example, in PCT application publication number WO2009/128034 with the additional step of admixing with the diamond grains, prior to sintering, catalyst material in the form of a metal binder such as 0 to 3 wt % cobalt. A powder blend comprising diamond particles and the metal binder material, such as cobalt may be prepared by combining these particles and blending them together. An effective powder preparation technology may be used to blend the powders, such as wet or dry multi-directional mixing, planetary ball milling and high shear mixing with a homogenizer. In one embodiment, the mean size of the diamond particles may be from about 1 to at least about 50 microns and they may be combined with other particles by mixing the powders or, in some cases, stirring the powders together by hand. In one version of the method, precursor materials suitable for subsequent conversion into binder material may be included in the powder blend, and in one version of the method, metal binder material may be introduced in a form suitable for infiltration into a green body. The powder blend may be deposited in a die or mold and compacted to form a green body, for example by uni-axial compaction or other compaction method, such as cold isostatic pressing (CIP). The green body may be subjected to a sintering process known in the art to form a sintered article. In one version, the method may include loading the capsule comprising a pre-sinter assembly into a press and subjecting the green body to an ultra-high pressure and a temperature at which the superhard material is thermodynamically stable to sinter the superhard grains.

After sintering, the polycrystalline super hard constructions may be ground to size and may include, if desired, a 45° chamfer of approximately 0.4 mm height on the body of polycrystalline super hard material so produced.

In the example of PCD, the sintered article may be subjected to a subsequent treatment at a pressure and temperature at which diamond is thermally stable to convert some or all of the non-diamond carbon back into diamond and produce a diamond composite structure. An ultra-high pressure furnace well known in the art of diamond synthesis may be used and the pressure may be at least about 5.5 GPa and the temperature may be at least about 1,250 degrees centigrade for the second sintering process.

A further embodiment of a superhard construction may be made by a method including providing a PCD structure and a precursor structure for a diamond composite structure, forming each structure into the respective complementary shapes, assembling the PCD structure and the diamond composite structure onto a cemented carbide substrate to form an unjoined assembly, and subjecting the unjoined assembly to a pressure of at least about 5.5 GPa and a temperature of at least about 1,250 degrees centigrade to form a PCD construction. The precursor structure may comprise carbide particles and diamond or non-diamond carbon material, such as graphite, and a binder material comprising a metal, such as cobalt. The precursor structure may be a green body formed by compacting a powder blend comprising particles of diamond or non-diamond carbon and particles of carbide material and compacting the powder blend.

In embodiments where the cemented carbide substrate does not contain sufficient solvent/catalyst for diamond, and where the PCD structure is integrally formed onto the substrate during sintering at an ultra-high pressure, solvent/catalyst material may be included or introduced into the aggregated mass of diamond grains from a source of the material other than the cemented carbide substrate. The solvent/catalyst material may comprise, for example, cobalt that infiltrates from the substrate into the aggregated mass of diamond grains just prior to and during the sintering step at an ultra-high pressure. However, in embodiments where the content of cobalt or other solvent/catalyst material in the substrate is low, particularly when it is less than about 11 weight percent of the cemented carbide material, then an alternative source may need to be provided in order to ensure good sintering of the aggregated mass to form PCD.

Solvent/catalyst for diamond may be introduced into the aggregated mass of diamond grains by various methods, including blending solvent/catalyst material in powder form with the diamond grains, depositing solvent/catalyst material onto surfaces of the diamond grains, or infiltrating solvent/catalyst material into the aggregated mass from a source of the material other than the substrate, either prior to the sintering step or as part of the sintering step. Methods of depositing solvent/catalyst for diamond, such as cobalt, onto surfaces of diamond grains are well known in the art, and include chemical vapour deposition (CVD), physical vapour deposition (PVD), sputter coating, electrochemical methods, electroless coating methods and atomic layer deposition (ALD). It will be appreciated that the advantages and disadvantages of each depend on the nature of the sintering aid material and coating structure to be deposited, and on characteristics of the grain.

In one embodiment, the binder/catalyst such as cobalt may be deposited onto surfaces of the diamond grains by first depositing a pre-cursor material and then converting the precursor material to a material that comprises elemental metallic cobalt. For example, in the first step cobalt carbonate may be deposited on the diamond grain surfaces using the following reaction:

Co(NO₃)₂+Na₂CO₃->CoCO₃+2NaNO₃

The deposition of the carbonate or other precursor for cobalt or other solvent/catalyst for diamond may be achieved by means of a method described in PCT patent publication number WO/2006/032982. The cobalt carbonate may then be converted into cobalt and water, for example, by means of pyrolysis reactions such as the following:

CoCO₃->CoO+CO₂

CoO+H₂->Co+H₂O

In another embodiment, cobalt powder or precursor to cobalt, such as cobalt carbonate, may be blended with the diamond grains. Where a precursor to a solvent/catalyst such as cobalt is used, it may be necessary to heat treat the material in order to effect a reaction to produce the solvent/catalyst material in elemental form before sintering the aggregated mass.

In some embodiments, the cemented carbide substrate may be formed of tungsten carbide particles bonded together by the binder material, the binder material comprising an alloy of Co, Ni and Cr. The tungsten carbide particles may form at least 70 weight percent and at most 95 weight percent of the substrate. The binder material may comprise between about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, and the remainder weight percent comprises Co.

Embodiments are described in more detail below with reference to the following example which is provided herein by way of illustration only and is not intended to be limiting.

EXAMPLE 1

An aggregated mass of diamond powder with an average grain size of 12 microns was ball milled in 60 ml of methanol with Co—WC milling balls. The ratio of milling balls:powder was 5:1 and milling was carried out for 1 hour at 90 rpm. Once milled, 2.1 g of the mixture was placed on top of a pre-formed WC—Co substrate. The pre-formed substrate has a projection extending to about 4 mm from the end surface of the substrate as shown in FIGS. 2(b) and 2(e). The substrate and mass of diamond powder were sintered under high pressure high temperature HPHT conditions at 5.5 GPa and 1450° C. to form a PCD cutter which was recovered, processed and analysed. The PCD cutter had a PCD thickness from the cutting surface to the interface with the substrate along the peripheral side edge of the cutter of around 4 mm.

The results of the analysis are discussed below with reference to FIG. 8.

Various sample of PCD material were prepared and analysed by subjecting the samples to a number of tests. The results of these tests are shown in FIG. 8.

The PCD compact formed according to Example 1 was compared in a vertical boring mill test with two leached conventional polycrystalline diamond cutter elements formed of diamond grains having an average grain size of 12 microns and which were sintered under pressures of around 5.5 GPa. The conventional PCD cutters in this test had non-planar interfaces and a thickness of the diamond table along the peripheral side edge of the cutter of around 2.5 mm. In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results obtained are illustrated graphically in FIG. 8. The results provide an indication of the total wear scar area plotted against cutting length. It will be seen that the PCD compact formed according to Example 1 denoted by the reference numeral 54 in FIG. 8, and having a diamond table thickness at the peripheral edge of the cutter of around 4 mm and projection from the substrate having a height of around 4 mm was able to achieve a greater cutting length and smaller wear scar area than that occurring in both of the conventionally leached PCD compacts (denoted by reference numerals 50 and 52) which were subjected to the same test for comparison.

Whilst not wishing to be bound by a particular theory, it is believed that crack propagation may be controlled by introducing a barrier material in the form of the substrate features to slow down the propagation rate of the crack before the critical length of the crack is reached and hence avoid spalling of the non-working area of the superhard material. The protrusion in the substrate has a higher impact resistance compared to the superabrasive layer and thereby acts to arrest the cracks to avoid spalling or catastrophic failure during use of the cutter element.

The size and shape of the substrate features may be tailored to the final application of the superhard material. It is believed possible to improve spalling resistance without significantly compromising the overall abrasion resistance of the material, which is desirable for PCD and PCBN cutting tools.

The vertical borer test results of these engineered structures show a considerable increase in PCD cutting tool life compared to conventional PCD, and with no degradation in abrasion resistance.

Observation of the wear scar development during testing showed the material's ability to generate large wear scars without exhibiting brittle-type micro-fractures (e.g. spalling or chipping), leading to a longer tool life.

Thus, embodiments of, for example, a PCD material, may be formed having a combination of high abrasion and fracture performance.

The PCD element 10 described with reference to FIG. 1 may be further processed after sintering. For example, catalyst material may be removed from a region of the PCD structure adjacent the working surface or the side surface or both the working surface and the side surface. This may be done by treating the PCD structure with acid to leach out catalyst material from between the diamond grains, or by other methods such as electrochemical methods. A thermally stable region, which may be substantially porous, extending a depth of at least about 50 microns or at least about 100 microns from a surface of the PCD structure, may thus be provided which may further enhance the thermal stability of the PCD element.

Furthermore, the PCD body in the structure of FIG. 1 comprising a PCD structure bonded to a cemented carbide support body may be created or finished by, for example, grinding, to provide a PCD element which is substantially cylindrical and having a substantially planar working surface, or a generally domed, pointed, rounded conical or frusto-conical working surface. The PCD element may be suitable for use in, for example, a rotary shear (or drag) bit for boring into the earth, for a percussion drill bit or for a pick for mining or asphalt degradation.

While various embodiments have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular embodiments disclosed. 

1. A superhard polycrystalline construction comprising: a body of polycrystalline superhard material having a cutting face; and a substrate bonded to the body of polycrystalline superhard material along an interface; the construction having a central longitudinal axis extending therethrough and a peripheral side edge; wherein: the substrate comprises a substrate body and a first end surface forming the interface, the first end surface of the substrate comprising a projection extending from the body of the substrate into the body of superhard material towards the cutting face, the projection having an outer peripheral surface, the body of polycrystalline superhard material extending around the peripheral outer surface of the projection; wherein the body of polycrystalline superhard material has a thickness from the cutting face along the peripheral side edge to the interface with the substrate of at least around 4 mm; and wherein at least a portion of the projection has a thickness measured in a plane extending along the longitudinal axis of the construction of at least around 4 mm.
 2. The superhard polycrystalline construction of claim 1, wherein the projection from the substrate extends to and forms part of the working face.
 3. The superhard polycrystalline construction of claim 1, wherein the projection from the substrate extends to a distance of around 0.5 mm or less from the cutting face.
 4. The superhard polycrystalline construction of claim 1, wherein the body of polycrystalline superhard material comprises natural and/or synthetic diamond grains, and/or cubic boron nitride grains. 5.-10. (canceled)
 11. The superhard polycrystalline construction of claim 1, wherein the body of superhard material comprises polycrystalline diamond material having interbonded diamond grains and interstices therebetween; wherein at least a portion of the body of superhard material is substantially free of a catalyst material for diamond, said portion forming a thermally stable region.
 12. The superhard polycrystalline construction of claim 11, wherein the depth of the thermally stable region from the cutting face along the peripheral side edge is at least around 3.5 mm or greater.
 13. The superhard polycrystalline construction of claim 11, wherein the depth of the thermally stable region from the cutting face along the peripheral side edge is at least around 4.5 mm or greater.
 14. The superhard polycrystalline construction of claim 1, further comprising a protective layer over at least a portion of the cutting face.
 15. (canceled)
 16. The superhard polycrystalline construction of claim 1, further comprising one or more interlayers bonded between at least a portion of the substrate and the body of superhard material.
 17. The superhard polycrystalline construction of claim 16, wherein the one or more interlayers differ from one or other or both of the other interlayers and/or the body of superhard material in grain size and/or composition.
 18. The superhard polycrystalline construction of claim 16, wherein one or more of the interlayers comprise(s) one or more of a mixture of WC and diamond powder(s), a mixture of cBN and diamond powder(s), and/or a mixture of refractory metal(s) and hard material powders, the hard material powders comprising one or more of tungsten, vanadium or molybdenum.
 19. (canceled)
 20. The superhard polycrystalline construction of claim 1, wherein the projection comprises a planar central portion spaced from the body of the substrate by an interconnecting surface.
 21. (canceled)
 22. The superhard polycrystalline construction of claim 20, wherein the interconnecting surface is concave.
 23. The superhard polycrystalline construction of claim 22, wherein the interconnecting surface extends from the planar central section to the peripheral side edge of the construction.
 24. (canceled)
 25. The superhard polycrystalline construction of claim 20, wherein the interconnecting surface comprises a first portion extending from the planar central section to a position spaced from the peripheral side edge of the construction, the interconnecting surface further comprising a second portion extending from the first portion to the peripheral side edge, the projection being substantially frustro-conical in shape.
 26. The superhard polycrystalline construction of claim 25, wherein the second portion forms a shoulder portion having a length of up to around 3 mm.
 27. The superhard polycrystalline construction of claim 25, wherein the peripheral outer surface of the projection is inclined at an angle of up to around 15 degrees from the central longitudinal axis.
 28. The superhard polycrystalline construction of claim 1, wherein the body of polycrystalline superhard material comprises an annular portion extending around the peripheral outer surface of the projection.
 29. The superhard polycrystalline construction of claim 28, wherein the annular portion is continuous around the outer peripheral surface of the projection.
 30. The superhard polycrystalline construction of claim 28, wherein the annular portion is discontinuous around the outer peripheral surface of the projection. 31.-62. (canceled) 